In typical prior art paper mills, an abundance of fresh water is needed for cooling and for washing requirements in the wire part and the press section of the paper machine and for dilution in the pulp preparation plant.
Fresh raw water is usually cold and fresh (non-salt) surface water, which is taken from rivers and/or lakes. This raw water must be purified chemically before use. Raw water is used, among other things, as supplementary water as well as for washing requirements in the production of pulp and paper, and it must be heated before use.
As is well known in the art of paper production, the production of pulp and paper consumes an abundance of thermal and electric energy, which is transferred into the circulation and cooling waters. The major part of the thermal energy that is lost is first in the form of steam, which is condensed by means of cold fresh water in condensers. A certain proportion of thermal energy is transferred to warm water in pulp and paper mills through various indirect heat exchangers. Of the warm water obtained from condensers and heat exchangers, only a part ends up in useful use, and the excess amount passes into the sewer. Owing to changes in the production of paper, the consumption of cold and warm fresh water varies, which causes changes in the properties of different waters, such as in its temperature and pH value, resulting in uneven quality of the pulp and paper produced therefrom. Cooling waters that flow into conventional biological treatment plants, which are situated on the way to the sewer, have a negative effect on the efficiency and costs of the treatment of the outgoing process and waste water.
As known in the prior art, the heated cooling water coming from condensers and heat exchangers is recirculated into an air cooling tower, in which the evaporating water is lost into the atmosphere. The outlet water from the cooling tower passes back into the condensers and heat exchangers. The evaporation and the growth of micro-organisms that take place in the cooling tower have the effect that chemical treatment, purification, and exchanging of the water are necessary in order to maintain the cooling efficiency.
The cooling tower is a heat exchanger for cooling of water, which cooling generally takes place by means of a direct contact between the cooling air and the liquid to be cooled. The cooling occurs in a manner such that the water is allowed to flow over a contact face as large as possible at the same time as this face is cooled by means of cooling air, which is introduced from outside the tower. The contact face consists of filler pieces and/or sets of plates in the cooling tower, and the function of these pieces or plates is to maximize the contact face between the cooling air and the water to be cooled. The principle of operation is primarily based on what is called "evaporative cooling", because the relatively high enthalpy of vaporization of water provides a relatively high takeoff of energy from the water phase even if the evaporation is not particularly abundant.
From the prior art, cooling towers are known which operate with a natural draught or with a cross-flow or counter-flow principle. Most often, the cooling towers are provided with blowers. When the cooling requirement is higher (larger flows of water), smaller units are often connected in parallel into larger units.